Karliah is a member of The Thieves' Guild and the Nightingales. She was forced on the run after she was framed by Mercer Frey for the murder of her lover and mentor Gallus Desidenius. She has spent the past 25 years plotting revenge on Mercer but her plans were foiled by the Dragonborn before they could come to fruition.
Karliah then enlists the help of the Dragonborn to track down Mercer and deal retribution for his betrayal of the Guild and for the murder of Gallus. At present, she has been reinstated as a full member of the Thieves Guild.